Fig 6. Model summarising the biophysical effects of NaCl stress on the leaf epidermis and its cell wall.

Treatment of maize plants for 8 days with 100 mM NaCl stress causes a size reduction of the epidermal cells derived from growth-inhibited maize leaves. This might be attributable to the epidermal cell walls being stiffer because of the reduced abundance of cell-wall-loosening β-expansin proteins. Under stress, the more resistant maize variety acidifies its epidermal apoplast to a pH range that is more favourable for acid growth and the activation of wall-loosening expansin proteins. In good agreement with this observation, the more resistant variety has a better capacity for epidermal cell expansion. The salinity-induced cell wall rheological modifications of the epidermis emphasize a contribution of the load-bearing epidermis in restricting the expansion of the entire leaf, ultimately contributing to the salinity-induced growth reduction.